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The Malpighian tubule system is a type of
The Malpighian tubule system is a type of excretory
Excretion is a process in which metabolic waste
is eliminated from an organism. In vertebrates this is primarily carried out by the lungs, kidneys, and skin. This is in contrast with secretion, where the substance may have specific tasks aft ...
and osmoregulatory system found in some insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three ...
s, myriapods
Myriapods () are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial.
The fossil record of myriapods reaches back into the late Silurian, ...
, arachnid
Arachnida () is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegar ...
s and tardigrade
Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbä ...
s.
The system consists of branching tubule
In biology, a tubule is a general term referring to small tube or similar type of structure. Specifically, tubule can refer to:
* a small tube or fistular structure
* a minute tube lined with glandular epithelium
* any hollow cylindrical body stru ...
s extending from the alimentary canal that absorbs solutes
In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...
, water, and wastes from the surrounding hemolymph
Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...
. The wastes then are released from the organism in the form of solid nitrogenous compounds and calcium oxalate. The system is named after Marcello Malpighi
Marcello Malpighi (10 March 1628 – 30 November 1694) was an Italian biologist and physician, who is referred to as the "Founder of microscopical anatomy, histology & Father of physiology and embryology". Malpighi's name is borne by several ph ...
, a seventeenth-century anatomist.
Structure
Malpighian tubules are slender tubes normally found in the posterior regions of arthropod alimentary canals. Each tubule consists of a single layer of cells that is closed off at the distal end with the proximal end joining the alimentary canal at the junction between the midgut and hindgut. Most tubules are normally highly convoluted. The number of tubules varies between species although most occur in multiples of two. Tubules are usually bathed inhemolymph
Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...
and are in proximity to fat body
300px, Stained cells of an insect fat body
Fat body is a highly dynamic insect tissue composed primarily of storage cells. It is distributed throughout the insect's internal body cavity; the haemocoel, in close proximity to the epidermis, dig ...
tissue. They contain actin
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ov ...
for structural support and microvilli for propulsion of substances along the tubules. Malpighian tubules in most insects also contain accessory musculature associated with the tubules which may function to mix the contents of the tubules or expose the tubules to more hemolymph. The insect orders, Dermaptera
Earwigs make up the insect order Dermaptera. With about 2,000 species in 12 families, they are one of the smaller insect orders. Earwigs have characteristic cerci, a pair of forcep-like pincers on their abdomen, and membranous wings folded ...
and Thysanoptera
Thrips (order Thysanoptera) are minute (mostly long or less), slender insects with fringed wings and unique asymmetrical mouthparts. Different thrips species feed mostly on plants by puncturing and sucking up the contents, although a few are ...
do not possess these muscles and Collembola
Springtails (Collembola) form the largest of the three lineages of modern hexapods that are no longer considered insects (the other two are the Protura and Diplura). Although the three orders are sometimes grouped together in a class called Ento ...
and Hemiptera:Aphididae
The Aphididae are a very large insect family in the aphid superfamily ( Aphidoidea), of the order Hemiptera. These insects suck the sap from plant leaves. Several thousand species are placed in this family, many of which are considered plant/crop ...
completely lack a Malpighian tubule system.
General mode of action
Pre-urine is formed in the tubules, whennitrogenous waste
Metabolic wastes or excrements are substances left over from metabolic processes (such as cellular respiration) which cannot be used by the organism (they are surplus or toxic), and must therefore be excreted. This includes nitrogen compounds, ...
and electrolytes
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon di ...
are transported through the tubule walls. Wastes such as urea
Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid.
Urea serves an important ...
and amino acids are thought to diffuse through the walls, while ions such as sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
and potassium
Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosph ...
are transported by active pump mechanisms. Water follows thereafter. The pre-urine, along with digested food, merge in the hindgut. At this time, uric acid
Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates, such as ammonium acid urate. Uric acid is a product of the metabolic breakdown ...
precipitates out, and sodium and potassium ions are actively absorbed by the rectum, along with water via osmosis
Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region o ...
. Uric acid is left to mix with feces, which are then excreted
Excretion is a process in which metabolic waste
is eliminated from an organism. In vertebrates this is primarily carried out by the lungs, kidneys, and skin. This is in contrast with secretion, where the substance may have specific tasks after lea ...
.
Alternative modes of action
Complex cycling systems of Malpighian tubules have been described in other insect orders. Hemipteran insects use tubules that permit movement of solutes into the distal portion of the tubules while reabsorption of water and essential ions directly to the hemolymph occurs in the proximal portion and the rectum. Both Coleoptera and Lepidoptera use a cryptonephridial arrangement where the distal end of the tubules are embedded in fat tissue surrounding the rectum. Such an arrangement may serve to increase the efficiency of solute processing in the Malpighian tubules.Other uses
Although primarily involved in excretion and osmoregulation, Malpighian tubules have been modified in some insects to serve accessory functions. Larvae of all species in genus ''Arachnocampa
''Arachnocampa'' is a genus of nine fungus gnat species which have a bioluminescent larval stage, akin to the larval stage of glowworm beetles. The species of ''Arachnocampa'' are endemic to Australia and New Zealand, dwelling in caves and grot ...
'' use modified and swollen Malpighian tubules to produce a blue-green lightGreen, L.B.S. (1979) The fine structure of the light organ of the New Zealand glow-worm ''Arachnocampa luminosa'' (Diptera: Mycetophilidae). ''Tissue and Cell'' 11: 457–465. attracting prey towards mucus-coated trap lines. In insects which feed on plant material containing noxious allelochemicals, Malpighian tubules also serve to rapidly excrete such compounds from the hemolymph.
See also
* CryptonephridiumReferences